1. Field of the Invention
This invention relates to television receiving apparatus having a picture-in-picture function and, more particularly, to such apparatus which is capable of receiving and concurrently displaying television programs that are broadcasted over different television channels by different television broadcast systems (such as television programs broadcasted over M, B/G, I, K or L systems).
2. Description of the Prior Art
Different television standards have arisen in different countries, particularly in the United States, Europe and Japan. Consequently, television receiving apparatus destined for a particular country must, of course, be compatible with the television standards adopted in that country. In most instances, because of the relatively limited range over which a broadcasted television signal may be received, and because generally uniform standards have been adopted in different geographic regions around the globe, the fact that different countries may broadcast television programs in accordance with different television standards does not present a serious problem. For example, one standard is used in North America (primarily the United States, Canada and Mexico), another standard is used in England, yet another standard is used in Western Europe, a further standard is used in France and a still other standard is used in Eastern Europe. For the most part, substantially the same standard has been adopted in Japan as is used in North America.
Television standards are based upon particular parameters of a typical television signal. For example, two basic standards have been developed around the world based upon the number of horizontal lines that are included in a video frame and the number of fields that are transmitted per second (i.e. the vertical frequency). These two broad standards are known as the 525/60 system, which is used primarily in North America and in Japan, and the 625/50 system, which is used primarily throughout Europe. Another parameter associated with particular television standards is the separation between the carrier frequency onto which the video information is modulated and the carrier frequency onto which the audio information is modulated. This channel separation between the video and audio broadcast frequencies is reflected in the difference between the video intermediate frequency and the audio intermediate frequency which are generated when a television program is received. This video/audio channel separation ranges from about 4.5 MHz in North America to about 6.5 MHz in Eastern Europe and in France.
Yet another parameter that is basic to a television standard is the manner in which color information is multiplexed for transmission. Typical of the color standards are the NTSC, PAL and SECAM standards. Normally, demodulating circuitry that is used to recover color information multiplexed in accordance with one standard is not readily usable to demodulate color information that has been multiplexed in accordance with a different standard.
A still further parameter that differs from one television standard to another is the specific intermediate frequency f.sub.p to which the broadcasted video signal is converted. As is known to those of ordinary skill in the art, although the broadcast frequency may change from one channel to the next (for example, in the United States there are twelve VHF frequencies and seventy UHF frequencies which comprise 82 broadcast channels), regardless of the particular channel to which a television receiver is tuned, only one video intermediate frequency is generated. This video IF frequency f.sub.p varies from, for example, 58.75 MHz in Japan to 45.75 MHz in the United States to 39.5 MHz in England.
There are primarily six different television standards used throughout the world. These standards have become known as the B, G, I, K, L and M standards; and the M standard is used in North America and Japan, with certain parameters of the M standard used in Japan being different from those parameters used in the United States, such as the video IF frequency mentioned above. For convenience, these standards are summarized in Table 1 as follows:
TABLE 1 __________________________________________________________________________ TV Standard (television broadcast system) B G I K L M __________________________________________________________________________ Number of 625 625 625 625 625 525 scanning lines Vertical 50 50 50 50 50 60 frequency (Hz) fs-fp (MHz) +5.5 +5.5 +6 +6.5 +6.5 +4.5 Difference -1.25 -1.25 -1.25 -1.25 -1.25 -1.25 between fp and adjacent channel (MHz) Channel band 7 8 8 8 8 6 width (MHz) Audio FM FM FM FM AM FM modulation system Popular name of West West U.K. East French American system European European systam European system system system system system __________________________________________________________________________
As used throughout this specification, the expression "television broadcast system" means the particular television standard which is used to create television signals that are broadcasted over a preselected broadcast channel.
Television signals are broadcasted in accordance with a single type of television broadcast system in a particular region, such as in Japan or in North America, and since the source of a different type of television broadcast system is so distant (e.g. Europe), it is highly unlikely, if not impossible, for a television receiver in the particular region to receive television signals broadcasted in accordance with a different system (such as from Europe). Thus, it is highly unlikely for a television receiver in the United States (which receives television signals in accordance with the M standard) to receive television signals broadcasted in accordance with the B, G, I, K or L standard. However, in Europe where heterogeneous television broadcast systems exist, it is likely that a television receiver in one country, such as in Germany, may receive television signals broadcasted by different television broadcast systems (as may emanate from France or Eastern Europe). That is, it is likely that such a television receiver may be able to pick up television signals broadcasted over different channels by B, G, K or L standards. Normally, the same broadcast frequency, or channel, is not used by two or more broadcasting stations that are sufficiently proximate to be picked up by one television receiver, as this would result in interference.
Rather than provide different television sets, each compatible with a respective television broadcast system, for use in those areas that are able to receive television signals broadcasted by different systems, it is desirable to provide a single television receiver having multiple standard capabilities. For example, it is desirable to manufacture a television receiver for use in Germany that is capable of receiving and displaying television programs broadcasted in accordance with B, G, K or L systems. Such a television receiver has been proposed, wherein different bandpass filters, trap filters and IF circuit operating characteristics, all consistent with different respective television broadcast systems, may be selected by the user. Of course, for proper selection of these filters and operating characteristics (referred to generally as demodulating parameters), the user should be provided with an indication of the type of television broadcast system that is being used to broadcast a television program over a particular broadcast channel.
Still further, with the recent introduction of television receivers having the so-called picture-in-picture function, it is desirable to display one picture of one television program broadcasted in accordance with one type of television broadcast system and simultaneously display, on the same display screen, yet another picture of another television program broadcasted in accordance with a different type of television broadcast system.
In picture-in-picture television receivers capable of receiving television programs broadcasted in accordance with different television broadcast systems, as proposed heretofore, identical television broadcast system identifying circuitry is provided in each demodulator that is used to produce a respective one of the "picture-in-picture" pictures. This results in a relatively expensive television receiver having complicated wiring and resulting in a device of relatively large size. Furthermore, identification of a particular type of television broadcast system being used to transmit the received television signals generally is dependent upon detecting the difference between the video and audio IF frequencies (Table 1 indicates that detection of this IF frequency difference provides a relatively accurate identification of the type of television broadcast system being used). However, when the broadcast channel to which one (or more) of the demodulators is tuned is changed, there is a substantial delay before the video and audio IF frequencies of that new channel are identified. Consequently, there is a delay in identifying the type of television broadcast system being used to transmit the television signals over this new channel and, as a result, a display of the corresponding video picture is delayed. The resultant blanking of the display screen is undesired and annoying.